For me it's a busy brain with any information it wants to consume at my finger tips. This is something I'm working incredibly hard to combat, but my brain is constantly going a million miles a minute with the most meaningless and out of my control crap. The way I can "close" the thought is by looking stuff up on the internet related to that thought, or just distracting myself with some garbage.

A calm mind to me at this point is one of the most valuable things. I seldomly experience it, but thankfully it's been happening more often lately now that I'm conscious about it.